python - forcing ctypes.cdll.LoadLibrary() to reload library from file

Question

I have the following code

import ctypes
lib1 = ctypes.cdll.LoadLibrary("./mylib.so")
# modify mylib.so (code generation and compilation) or even delete it
lib2 = ctypes.cdll.LoadLibrary("./mylib.so")

The problem is that lib2 refers to the original shared library, not the new one. If I delete mylib.so between the calls I get no error.

Using ctypes._reset_cache() does not help.

How can I tell ctypes to actually reload the library from the hard disk?

Answer 1

I don't know how to instruct ctypes how to unload a library (didn't find a way on [Python 3]: ctypes - A foreign function library for Python, but that doesn't mean that there isn't one).

It can be done manually, by forcing the loader to (decrement the library's reference count and) unload it via [man7]: DLCLOSE(3P) (also read [man7]: DLOPEN(3) for additional info on loading / unloading libraries).

dll.c:

#include <stdio.h>

int func0(int arg0) {
    int alter_factor = 2;
    printf("From C - arg0: %d, alter_factor: %d
", arg0, alter_factor);
    return arg0 * alter_factor;
}

code.py:

#!/usr/bin/env python3

import sys
import ctypes


DLL_NAME = "./dll.so"


def handle_dll(dll_name=DLL_NAME):
    dll_dll = ctypes.CDLL(dll_name)
    func0_func = dll_dll.func0
    func0_func.argtypes = [ctypes.c_int]
    func0_func.restype = ctypes.c_int
    return dll_dll, func0_func


def main():
    dlclose_func = ctypes.CDLL(None).dlclose
    dlclose_func.argtypes = [ctypes.c_void_p]
    dlclose_func.restype = ctypes.c_int

    dll, func0 = handle_dll()
    res = func0(42)
    print(res)
    dlclose_func(dll._handle)
    input("In another terminal, modify the C code (e.g. change `alter_factor`), recompile (gcc -fPIC -shared -o dll.so dll.c), and when done press ENTER here...")
    dll, func0 = handle_dll()
    res = func0(42)
    print(res)


if __name__ == "__main__":
    print("Python {:s} on {:s}
".format(sys.version, sys.platform))
    main()

Output:

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050964033]> python3 code.py
Python 3.5.2 (default, Nov 23 2017, 16:37:01)
[GCC 5.4.0 20160609] on linux

From C - arg0: 42, alter_factor: 2
84
In another terminal, modify the C code (e.g. change `alter_factor`), recompile (gcc -fPIC -shared -o dll.so dll.c), and when done press ENTER here...
From C - arg0: 42, alter_factor: 3
126